Degrading Water Quality and Shrinking Usable Supply.

 Increasing scarcity of water is not only a story of declining volumes. It is equally a story of water that is physically present but no longer fit for purpose. Globally, a large share of municipal and industrial wastewater is still released to the environment untreated or only partially treated, while diffuse pollution from agriculture—nutrients, pesticides, pathogens and sediments—degrades rivers, lakes, reservoirs and coastal waters. In many rapidly urbanizing basins, downstream communities face a hydrological paradox: river flows may be sufficient in purely volumetric terms, yet much of that water cannot be used safely for drinking, irrigation, or recreation without costly treatment and poses serious risks to human and ecosystem health. Groundwater systems face similar pressures. Nitrate contamination from fertilizers and livestock, industrial pollutants, and naturally occurring arsenic andfluoride mobilized by deeper pumping have rendered some aquifers unsafe for drinking water, even before accounting for depletion. Saltwater intrusion into coastal aquifers, driven by over-abstraction and sealevel rise, can permanently compromise groundwater quality and force expensive alternative supply options or relocation of users. These dynamics mean that the effective, usable balance sheet of water is shrinking faster than simple volume statistics suggest. Any realistic diagnosis of water scarcity must therefore integrate both quantity and quality, recognizing that degraded water quality reduces the real carrying capacity of human–water systems and can be as bad as physical depletion.

State of access to safe drinking water and improved sanitation services around the world. The top map shows the percentage of the population with no access to safe drinking water supplies, collecting drinking water from unprotected and unsafe sources. The map on the bottom shows the percentage of population without access to improved sanitation services, including those who directly dispose human waste in open bodies of water, beaches, forests, and other open spaces. Maps produced based on Aqueduct 4.0 data.

State of surface water quality across the world. The top map shows annual average Biochemical Oxygen Demand (BOD) concentrations for the 2010–2019 period, indicative of organic pollution levels; high BOD means more organic pollution, depleting vital dissolved oxygen for aquatic life, potentially harming ecosystems. The middle map illustrates Total Dissolved Solids (TDS) concentrations for the 2010–2019 period, indicative of salinity levels and dissolved minerals; high TDS means more dissolved substances and pollution, potentially impacting water quality and utility. The bottom map displays Fecal Coliform (FC) concentrations for the 2010–2019 period, indicative of fecal contamination from human or animal waste; high FC counts mean a greater likelihood of harmful pathogens, increasing the risk of waterborne diseases. Maps produced based on data from Jones et al. (2023), Geoscientific Model Development.